Seabed logging data are traditionally acquired along a line and can often be compared and interpreted directly. When data is acquired with different geometry and the source is no longer pointing towards the receiver this interpretation can be difficult. We consider an azimuth decomposition which takes into account the 3D acquisition geometry and that recovers the inline response under a plane layer assumption. We show that this approach can be used when this assumption is violated. However, the azimuth angle must be restricted under such conditions.
In 2002 Eidesmo et al. and Ellingsrud et al. introduced a particular application of controlled marine source electromagnetic sounding called seabed logging (SBL). The SBL technique was introduced as a tool for hydrocarbon exploration, also in shallow waters (Johnstad et al., 2005). In particular it is very sensitive to thin resistive layers buried in the subsurface. This sensitivity relies on two important properties. First, the distance between the source and the receiver must be sufficiently large to obtain a response from the resistive layer. Second, a vertical component of the electric field must be present in the subsurface. When the vertical electric field enters a horizontal high resistive layer the amplitude will rise significantly due to the conservation of the electric current normal to the interface. This makes it possible to excite a partly guided event along a thin horizontal high resistive layer. A horizontal electric dipole is used as a source and is towed along the seafloor. The radiation pattern from this source is such that only the transverse electric field is excited in the direction perpendicular to the source (TE mode), and hence gives no or little sensitivity to thin horizontal high resistive layers. On the other hand, only a transverse magnetic field is excited in the direction along the source (TM mode). In a horizontally layered structure the response from the TM mode and the response from the TE mode will not mix at any source and receiver position due to rotational and translational symmetry. A source with arbitrary orientation (azimuth) with respect to the sourcereceiver (inline) direction can therefore be decomposed into its TE mode and its TM mode. Such decomposition can be very useful as data acquired with different azimuth can be directly compared. This can be of great importance for interpretation of target specific acquisition (as shown later), and when data are acquired on sparsely sampled receiver grids for scanning of large areas. In the following we show examples of this and compare data with different azimuth under conditions which breaks the symmetry assumptions. We will focus the comparison to consider data due to the TM mode. For synthetic data we compare the resulting measured electric fields with a source pointing in the true inline direction. Also a real data example from the North Sea is given.
The horizontal components of the electric source are decomposed into one TM mode component and one TE mode component.